Prions are infectious pathogens that cause central nervous system spongiform encephalopathies in humans and animals. Prions are distinct from bacteria, viruses and viroids. The predominant hypothesis at present is that no nucleic acid component is necessary for infectivity of prion protein. Further, a prion which infects one species of animal (e.g., a human) will not infect another (e.g., a mouse).
A major step in the study of prions and the diseases that they cause was the discovery and purification of a protein designated prion protein ("PrP") [Bolton et al., Science 218: 1309-11 (1982); Prusiner et al., Biochemistry 21: 6942-50 (1982); McKinley et al., Cell 35: 57-62 (1983)]. Complete prion protein-encoding genes have since been cloned, sequenced and expressed in transgenic animals. PrP.sup.C is encoded by a single-copy host gene [Basler et al., Cell 46: 417-28 (1986)] and is normally found at the outer surface of neurons. A leading hypothesis is that prion diseases result from conversion of PrP.sup.C into a modified form called PrP.sup.Sc. However, the actual biological or physiological function of PrP.sup.C is not known.
It appears that the scrapie isoform of the prion protein (PrP.sup.Sc) is necessary for both the transmission and pathogenesis of the transmissible neurodegenerative diseases of animals and humans. See Prusiner, S. B., "Molecular biology of prion disease," Science 252: 1515-1522 (1991). The most common prion diseases of animals are scrapie of sheep and goats and bovine spongiform encephalopathy (BSE) of cattle [Wilesmith, J. and Wells, Microbiol. Immunol. 172: 21-38 (1991)]. Four prion diseases of humans have been identified: (1) kuru, (2) Creutzfeldt-Jakob Disease (CJD), (3) Gerstmann-Strassler-Scheinker Disease (GSS), and (4) fatal familial insomnia (FFI) [Gajdusek, D. C., Science 197: 943-960 (1977); Medori et al., N. Engl. J. Med. 326: 444-449 (1992)]. The presentation of human prion diseases as sporadic, genetic and infectious illnesses initially posed a conundrum which has been explained by the cellular genetic origin of PrP.
Most CJD cases are sporadic, but about 10-15% are inherited as autosomal dominant disorders that are caused by mutations in the human PrP gene [Hsiao et al., Neurology 40: 1820-1827 (1990); Goldfarb et al., Science 258: 806-808 (1992); Kitamoto et al., Proc. R. Soc. Lond. (In press) (1994)]. Iatrogenic CJD has been caused by human growth hormone derived from cadaveric pituitaries as well as dura mater grafts [Brown et al., Lancet 340: 24-27 (1992)]. Despite numerous attempts to link CJD to an infectious source such as the consumption of scrapie infected sheep meat, none has been identified to date [Harries-Jones et al., J. Neurol. Neurosurg. Psychiatry 51: 1113-1119 (1988)] except in cases of iatrogenically induced disease. On the other hand, kuru, which for many decades devastated the Fore and neighboring tribes of the New Guinea highlands, is believed to have been spread by infection during ritualistic cannibalism [Alpers, M. P., Slow Transmissible Diseases of the Nervous System, Vol. 1, S. B. Prusiner and W. J. Hadlow, eds. (New York: Academic Press), pp. 66-90 (1979)].
The initial transmission of CJD to experimental primates has a rich history beginning with William Hadlow's recognition of the similarity between kuru and scrapie. In 1959, Hadlow suggested that extracts prepared from patients dying of kuru be inoculated into non-human primates and that the animals be observed for disease that was predicted to occur after a prolonged incubation period [Hadlow, W. J., Lancet 2: 289-290 (1959)]. Seven years later, Gajdusek, Gibbs and Alpers demonstrated the transmissibility of kuru to chimpanzees after incubation periods ranging form 18 to 21 months [Gajdusek et al., Nature 209: 794-796 (1966)]. The similarity of the neuropathology of kuru with that of CJD [Klatzo et al., Lab Invest. 8: 799-847 (1959)] prompted similar experiments with chimpanzees and transmissions of disease were reported in 1968 [Gibbs, Jr. et al., Science 161: 388-389 (1968)]. Over the last 25 years, about 300 cases of CJD, kuru and GSS have been transmitted to a variety of apes and monkeys.
The expense, scarcity and often perceived inhumanity of such experiments have restricted this work and thus limited the accumulation of knowledge. While the most reliable transmission data has been said to emanate from studies using non-human primates, some cases of human prion disease have been transmitted to rodents but apparently with less regularity [Gibbs, Jr. et al., Slow Transmissible Diseases of the Nervous System, Vol. 2, S. B. Prusiner and W. J. Hadlow, eds. (New York: Academic Press), pp. 87-110 (1979); Tateishi et al., Prion Diseases of Humans and Animals, Prusiner et al., eds. (London: Ellis Horwood), pp. 129-134 (1992)].
The infrequent transmission of human prion disease to rodents has been cited as an example of the "species barrier" first described by Pattison in his studies of passaging the scrapie agent between sheep and rodents [Pattison, I. H., NINDB Monograph 2, D.C. Gajdusek, C. J. Gibbs Jr. and M. P. Alpers, eds. (Washington, D.C.: U.S. Government Printing), pp. 249-257 (1965)]. In those investigations, the initial passage of prions from one species to another was associated with a prolonged incubation time with only a few animals developing illness. Subsequent passage in the same species was characterized by all the animals becoming ill after greatly shortened incubation times.
The molecular basis for the species barrier between Syrian hamster (SHa) and mouse was shown to reside in the sequence of the PrP gene using transgenic (Tg) mice [Scott et al., Cell 59: 847-857 (1989)]. SHaPrP differs from MoPrP at 16 positions out of 254 amino acid residues [Basler et al., Cell 46: 417-428 (1986); Locht et al., Proc. Natl. Acad. Sci. USA 83: 6372-6376 (1986)]. Tg(SHaPrP) mice expressing SHaPrP had abbreviated incubation times when inoculated with SHa prions. When similar studies were performed with mice expressing the human, or ovine PrP transgenes, the species barrier was not abrogated, i.e., the percentage of animals which became infected were unacceptably low and the incubation times were unacceptably long. Thus, it has not been possible, for example in the case of human prions, to use transgenic animals (such as mice containing a PrP gene of another species) to reliably test a sample to determine if that sample is infected with prions. Such a test was first disclosed in parent application Ser. No. 08/242,188 filed May 13, 1994 which is now U.S. Pat. No. 5,565,186 issued Oct. 15, 1996.
More than 45 young adults previously treated with HGH derived from human pituitaries have developed CJD [Koch et al., N. Engl. J. Med. 313: 731-733 (1985); Brown et al., Lancet 340: 24-27 (1992); Fradkin et al., JAMA 265: 880-884 (1991); Buchanan et al., Br. Med. J. 302: 824-828 (1991)]. Fortunately, recombinant HGH is now used, although the seemingly remote possibility has been raised that increased expression of wtPrP.sup.C stimulated by high HGH might induce prion disease [Lasmezas et al., Biochem. Biophys. Res. Commun. 196: 1163-1169 (1993)]. That the HGH prepared from pituitaries was contaminated with prions is supported by the transmission of prion disease to a monkey 66 months after inoculation with a suspect lot of HGH [Gibbs, Jr. et al., N. Engl. J. Med. 328: 358-359 (1993)]. The long incubation times associated with prion diseases will not reveal the full extent of iatrogenic CJD for decades in thousands of people treated with HGH worldwide. Iatrogenic CJD also appears to have developed in four infertile women treated with contaminated human pituitary-derived gonadotrophin hormone [Healy et al., Br. J. Med. 307: 517-518 (1993); Cochius et al., Aust. N. Z. J. Med. 20: 592-593 (1990); Cochius et al., J. Neurol. Neurosurg. Psychiatry 55: 1094-1095 (1992)] as well as at least 11 patients receiving dura mater grafts [Nisbet et al., J. Am. Med. Assoc. 261: 1118 (1989); Thadani et al., J. Neurosurg. 69: 766-769 (1988); Willison et al., J. Neurosurg. Psychiatric 54: 940 (1991); Brown et al., Lancet 340: 24-27 (1992)]. These cases of iatrogenic CJD underscore the need for screening pharmaceuticals that might possibly be contaminated with prions.
Recently, two doctors in France were charged with involuntary manslaughter of a child who had been treated with growth hormones extracted from corpses. The child developed Creutzfeldt-Jakob Disease. (See New Scientist, Jul. 31, 1993, page 4). According to the Pasteur Institute, since 1989 there have been 24 reported cases of CJD in young people who were treated with human growth hormone between 1983 and mid-1985. Fifteen of these children have died. It now appears as though hundreds of children in France have been treated with growth hormone extracted from dead bodies at the risk of developing CJD (see New Scientist, Nov. 20, 1993, page 10.) Investigations of the prion diseases have taken on new significance with the reports of more than 20 cases of an atypical, variant CJD (vCJD) in teenagers and young adults [D. Bateman et al., Lancet 346: 1155 (1995); T. C. Britton, S. Al-Sarraj, C. Shaw, T. Campbell, J. Collinge, Lancet 346: 1155 (1995); G. Chazot et al., Lancet 347: 1181 (1996); R. G. Will et al., Lancet 347: 921 (1996); S. N. Cousens, E. Vynnyoky, M. Zeidler, R. G. Will, P. G. Smith, Nature 385: 197 (1997)]. To date, all of these cases have been reported from Great Britain and France. It now seems possible that bovine prions from "mad cows" passed to humans through the consumption of tainted beef products. It is generally thought that prion contaminated offal initially from sheep and later from cattle was used in the manufacture of meat and bone meal (MBM), and that this is the source of prions responsible for BSE [J. W. Wilesmith, J. B. M. Ryan, M. J. Atkinson, Vet. Rec. 128: 199 (1991); N. Nathanson, J. Wilesmith, C. Griot, Am. J. Epidemiol. 145: 959 (1997)].
Understanding the species barrier is paramount in our efforts to evaluate the impact of the BSE epidemic in Britain on human health [R. M. Anderson et al., Nature 382: 779 (1996)]. It has been estimated that almost one million cattle were infected with BSE prions with an incubation time of about 5 years. This may be an underestimation of the disease incidence as most cattle were slaughtered between 2 and 3 years of age [D. J. Stekel, M. A. Nowak, T.R.E. Southwood, Nature 381: 119 (1996)]. Nevertheless, more than 160,000 cattle, primarily dairy cows, have died of BSE over the past decade. In the late 1970s, the hydrocarbon-solvent extraction method used in the rendering of offal began to be abandoned resulting in MBM with a much higher fat content. It is now thought that this change in the rendering process allowed scrapie prions from sheep to survive rendering and to be passed into cattle [J. W. Wilesmith, Semin. Virol. 2: 239 (1991); R. H. Kimberlin, Bovine Spongiform Encephalopathy: The BSE Dilemma C. J. Gibbs, Jr., Ed. (Springer, New York, 1996) pp. 155-175].
Although many plans have been offered for the culling of older cattle in order to minimize the spread of BSE, it seems more important to monitor the frequency of prion disease in cattle as they are slaughtered for human consumption. No completely reliable, specific test for prion disease in live animals is available [G. Hsich, K. Kenney, C. J. Gibbs, K. H. Lee, M. G. Harrington, N Engl. J. Med. 335: 924 (1996)], but immunoblotting of the brainstems of cattle for PrP.sup.Sc might provide a reasonable approach to establish the incidence of subclinical BSE in cattle entering the human food chain [J. Hope et al., Nature 336: 390 (1988); D. Serban, A. Taraboulos, S. J. DeArmond, S. B. Prusiner, Neurology 40: 110 (1990); A. Taraboulos et al., Proc. Natl. Acad. Sci. USA 89: 7620 (1992); S.B. Prusiner et al., J. Infect. Dis. 167: 602 (1993); K.-U. D. Grathwohl, M. Horiuchi, N. Ishiguro, M. Shinagawa, J. Virol. Methods 64: 205 (1997)]. Determining how early in the incubation period PrP.sup.Sc can be detected by immunological methods is complicated by the lack of a reliable, sensitive, and relatively rapid bioassay.